CD40 ligand (CD40L), also known as CD154, gp39, TNF-related activation protein (TRAP), 5c8 antigen, or T-BAM, is a trimeric transmembrane protein of the tumor necrosis factor (TNF) superfamily of molecules. CD40L is primarily expressed on activated T cells, as well as on activated leukocytes, cosinophils, basophils, natural killer cells, mast cells, and non-immune cells such as platelets and activated endothelial cells. CD40L also exists in soluble form (sCD40L) that is produced by microsomal stimulus-dependent cleavage of the membrane-bound CD40L. Most of sCD40L in circulation (>90%) is platelet-derived.
CD40L binds CD40, a type I transmembrane glycoprotein belonging to the TNF receptor (TNFR) family. Although all monomeric, dimeric, and trimeric forms of sCD40L can bind to CD40, the trimeric form of sCD40L has the most potent biological activity through oligomerization of cell surface CD40, a common feature of TNFR family. The highest expression of CD40 has been observed on antigen presenting cells (APCs), such as B cells, macrophages, and dendritic cells, while lower expression of this receptor is noted on a variety of other cell types, including stromal cells and thymic epithelium. The CD40-CD40L interaction is essential for normal T-B cell interactions, including increased co-stimulation, T-cell priming, cytokine production, antibody-class switching and affinity maturation, and antibody and autoantibody production.
The crucial role of CD40-CD40L interactions in immune and inflammatory responses has made them a promising target for treatment of pathological immuno-inflammatory processes. Blockade of CD40-CD40L interactions by means of specific CD40L monoclonal antibodies (mAbs) successfully prevents allograft rejection in primates and treats autoimmune diseases and atherosclerosis in animal models. Montgomery et al., Transplantation 74: 1365-1369 (2002).
In humans, two different anti-CD40L mAb clones have been used in clinical trials for treatment of different autoimmune diseases. Maribel et al., Mol. Immunol. 45: 937-44 (2008). Monoclonal antibodies, however, can display unusually high incidence of thromboembolic (TE) complications, such as atherothrombotic central nervous system events, myocardial infarction, pulmonary embolism, and deep vein thrombosis. For example, the usefulness of the anti-CD40L mAb clone hu5c8 (anti-CD40L mAb, Biogen) is limited by an unusually high incidence of TE complications. TE by these antibodies is thought to result from the formation of higher-order immune complexes (IC) of the mAbs with membrane-bound CD40L on platelets, or sCD40L shed from platelets, that can ligate and thereby aggregate neighboring platelets via their FcgRIIa receptors, resulting in thrombi formation. The risk of thromboembolism has led to a halt in all ongoing clinical trials. Boumpas et al., Arthritis & Rheumatism 48: 719-727 (2003).